1. Field of the Invention
The present invention relates to position/ime detection apparatus using a GPS (Global Positioning System) and timepieces using the detecting apparatus.
2. Descriptions of the Related Art
Measurement of a current position a correct time of a device using a GPS is performed in many fields for example, of car navigation.
GPS wristwatches have been proposed which each receive radio waves from GPS satellites to detect its position. As shown in FIG. 12, this wristwatch 20 comprises a timepiece circuit 11, a GPS receiver 12 which receives the GPS radio waves, a memory 13 which has a city information storage area 14, an operation unit 15 including various input keys, a display 16 which includes a liquid crystal display, a controller 17 and a power supply 18.
The city information storage area 14 of the memory 13 has stored data on the latitudes and longitudes of main cities in the world and on time differences of those cities from their associated standard times. The memory 13 further has another area which has stored programs to realize various functions of the GPS timepiece 20, and data produced as results of various processing operations performed by the controller 17.
The controller 17 displays on the display 16 time counted by the timepiece circuit 11. In response to an input from the operation unit 15 or at predetermined intervals of time, the controller 17 causes the GPS receiver 12 to receive the GPS radio waves and acquires information included in the GPS radio waves. The controller 17 calculates a current time and a current position of the GPS timepiece 20 based on the information acquired from the GPS receiver 17. In addition, the controller 17 compares the latitude and longitude of the calculated current position of the timepiece 20 with the latitudes and longitudes of the respective cities stored in the city information storage area 14 to locate a city where the GPS timepiece 20 is positioned at present and calculates a current time of the city, additionally using a time difference of the located city. The controller 17 then adjusts the time of the timepiece circuit 11 based on the calculated current time and then displays the corrected current time on the display 16.
The GPS receiver 12 operates in response to the operation unit 15 or at predetermined intervals of time. The position measurement of the GPS timepiece 20 is usually performed by receiving radio waves from four GPS satellites. The GPS radio waves include information on the orbits of these GPS satellites and information on GMT (Greenwich Mean Time) clock times at these GPS satellites when these respective satellites emitted the radio waves. This GMT clock time can be hereinafter referred to as a radio-wave emission GMT clock time. The GPS timepiece 20 calculates three dimensional coordinates (x, y, z) of the current position thereof and a time error δ involved in the current time of the GPS timepiece based on these information.
Situations occur often where it is difficult to acquire the orbit information and the radio-wave emission GMT clock time information on the four respective GPS satellites. Published Unexamined Japanese Patent Application No. 2007-256041 discloses a navigation device which includes a GPS receiver which detects changes in the environment where the GPS measurement is performed and in the altitude of the navigation device above sea level, thereby switching between 2- and 3-dimensional position measurements. When the 2-dimensional measurement is employed as the environment where the GPS measurement is performed deteriorates, the number of GPS satellites from which the GPS radio waves should be acquired is reduced to 3.
Especially, with a small GPS timepiece, for example, of a wristwatch type which employs a battery as a power supply, reduction in the power consumption is required. In order to obtain a correct current time and a correct current position of the timepiece periodically, the GPS receiver is required to be operated periodically. The current power consumption of the GPS receiver is high, or tens of milliwatt. Thus, when the GPS receiver is operated in the environment where the intensity of the received GPS radio waves is low, a long time is required to acquire information on the orbits and the radio wave-emission GMT clock times of the GPS satellites and as the case may be, the reception can fail. That is, with a small device such as uses a battery as its power supply, it is very important to avoid an undesirable situation in which satisfactory measurement of the position of the GPS timepiece cannot be obtained in spite of consumption of much power from the battery.
It is therefore an object of the present invention to provide a position/ime detecting apparatus capable of measuring its current position and current time in an appropriate manner by suppressing useless consumption of power and a timepiece including the position/time detecting apparatus.